The invention relates to a vacuum brake booster comprising a vacuum chamber and a working chamber separated from each other by a movable wall, a control valve which comprises a housing coupled workingly to said movable wall and which is capable of controlling the supply of atmospheric pressure or above-atmospheric pressure to the working chamber to achieve a pressure difference at the movable wall, and an emergency braking aid having a permanent magnet, which is disposed in the control valve housing, and an armature, which cooperates with the permanent magnet and in the event of emergency braking is drawn into abutment with the permanent magnet, with the result that the control valve is held open for the supply of atmospheric pressure or above-atmospheric pressure to the working chamber. The permanent magnet and the armature form a two-component magnetic module wherein a first component of the magnetic module is rigidly coupled to an actuating piston in actuating direction and a second component of the magnetic module is adapted to be coupled to the actuating piston at least in the actuating direction of the brake booster. In the context of the present invention, the term xe2x80x9cactuating directionxe2x80x9d always means the actuating direction of the brake booster.
Vacuum brake boosters have been known for quite some time and millions of them are being used to boost the actuating forces of a vehicle hydraulic brake system and therefore keep said forces at a level which is acceptable to the driver of a vehicle. Likewise known are so-called emergency braking aids, which are frequently also referred to as xe2x80x9cbrake assistantsxe2x80x9d. These are devices which provide a driver in the event of an emergency braking situation with increased braking power for substantially the same actuating force.
Emergency braking aids may be divided into electro-magnetically actuated and mechanically actuated systems. For reasons of cost, the use of a mechanical system is desired for applications in vehicles at the lower end of the price range.
A vacuum brake booster having such a mechanical emergency braking aid is known, for example, from WO 00/07862, corresponding U.S. Patent Application Publication 2001/0003947 A1 which is incorporated by reference herein. Said vacuum brake booster has a vacuum chamber and a working chamber separated from each other in a pressure-proof manner by a movable wall. A control valve, which has a housing coupled workingly to the movable wall, comprises an atmospheric valve seat which, to achieve a pressure difference at the movable wall, is capable of controlling the supply of atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster. The input element is coupled in actuating direction to an actuating piston.
For improved boosting of the braking force in emergency braking situations, a mechanical emergency braking aid is disposed in the control valve housing. The emergency braking aid includes a two-component armature comprised of a permanent magnet and an armature. In the actuating direction, the armature cooperating with the permanent magnet is rigidly coupled to the input element via the actuating piston. A coupling device makes it possible to couple the armature to the permanent magnet in such a manner that the axial distance between the armature and the permanent magnet does not change as a braking operation commences.
The armature is resiliently preloaded counter to the actuating direction of the brake booster and, in the starting position of the control valve, is held at a first distance from the permanent magnet. In the course of an approach towards the permanent magnet, the armature, when it is less than a predetermined second distance away, which is smaller than the first distance, is pulled by the permanent magnet counter to the resilient preloading force acting upon the armature and with simultaneous cancellation of its, in actuating direction, rigid coupling to the input element into abutment with the permanent magnet.
The movement of the armature is transmitted to a valve sleeve, which is rigidly coupled to the armature and on its end facing the input element carries the atmospheric valve seat. When the emergency braking aid is activated, because of the coupling of armature and permanent magnet, the atmospheric valve is held open to the maximum extent. The maximum possible pressure difference therefore builds up, with the result that the maximum possible boosting force of the brake booster is achieved.
To deactivate an activated emergency braking aid, the actuating force summoned up by the driver has to be reduced. As a result of the reduction of the actuating force summoned up by the driver, the actuating piston moves counter to actuating direction and a catch rigidly coupled to the actuating piston separates the armature from the permanent magnet.
So long as the full-output pressure of the brake booster, i.e. the maximum pressure difference at the movable wall, is not attained, a defined actuating speed excess is needed to move the armature closer than the second distance to the permanent magnet and hence achieve the coupling of armature and permanent magnet. However, once the full-output pressure is attained, a further increase of the actuating force summoned up by the driver is, independently of the actuating speed, always combined with an approach of armature and permanent magnet. Thus, even if the actuating force is increased slowly, e.g. when a vehicle is stopped at traffic lights, after the full-output pressure is attained an inappropriate coupling of armature and permanent magnet may occur. The emergency braking aid is activated even though an emergency braking situation does not exist.
To deactivate the inappropriately activated emergency braking aid, the driverxe2x80x94just as in the case of appropriate activation after emergency brakingxe2x80x94has to ease off the brake pedal to a relatively large extent before the brake booster drops back to its original performance characteristic and may once more be apportioned in the usual manner by the driver.
The driver is not accustomed to the departure from the usual performance characteristic which occurs upon activation of the emergency braking aid in non-emergency braking situations and he therefore perceives it to be a disadvantage.
The object of the invention is to provide a brake booster with a mechanical emergency braking aid, in which activation of the emergency braking aid may be effected only in emergency braking situations.
Proceeding from a brake booster of the type described initially, said object is achieved according to the invention in that a coupling device is provided, which may couple the second, as yet uncoupled component of the magnetic module at least in actuating direction of the brake booster to the actuating piston, when increased reaction forces are introduced counter to the actuating direction of the brake booster into the coupling device.
In a non-emergency braking situation, i.e. in the case of e.g. a slow increase of the actuating force, both the armature and the permanent magnet are coupled at least in actuating direction to the actuating piston by the coupling device. As a result of the simultaneous coupling of both armature and permanent magnet to the actuating piston the mutual distance of the two components of the magnetic module cannot be reduced any further. Armature and permanent magnet are therefore prevented from moving so close to one another that they are less than the previously mentioned second distance apart and a coupling of armature and permanent magnet occurs.
According to the invention, the hydraulic reaction forces of the master brake cylinder acting counter to actuating direction upon the actuating piston are used as a criterion for the existence of an emergency braking situation. Given a comparatively slow increase of the actuating force summoned up by the driver, i.e. in a non-emergency braking situation, the reaction forces of the master brake cylinder are relatively high. In said case, as a response to the high reaction forces, so to say, the coupling device is activated by them and the magnetic module component not yet coupled to the actuating piston is coupled, at least in actuating direction, to the actuating piston. An inappropriate activation of the emergency braking aid is ruled out in this event.
In the case of a fast actuation of the brake system which is typical of emergency braking situations, on the other hand, the reaction forces of the master brake cylinder are initially still comparatively low. The coupling device consequently remains deactivated and the emergency braking aid may cut in in the usual manner.
According to a preferred embodiment, a mechanical coupling device is provided. The reaction forces of the master brake cylinder advantageously act counter to actuating direction upon the mechanical coupling device so that the second, as yet uncoupled component of the magnetic module is coupled at least in actuating direction to the actuating piston, e.g. when the reaction forces exceed a specific threshold. The coupling device is in said case preferably disposed workingly between the actuating piston and a component of the brake booster which introduces the reaction forces of the master brake cylinder into the brake booster. According to the principle xe2x80x9cactio=reactioxe2x80x9d, the coupling device is then acted upon not only by the actuating force acting in actuating direction summoned up by the driver but also by the reaction force of the master brake cylinder acting counter to the actuating direction.
The coupling of the magnetic module component, which is not yet coupled to the actuating piston, is preferentially effected by means of a clamping joint to the actuating piston. But different constructions of the coupling device are also possible. For example, the coupling device may comprise latch or catch elements to couple the still uncoupled magnetic module component to the actuating piston.
In case a clamping connection is provided for coupling the still uncoupled component of the magnetic module component to the actuating piston, the coupling device may comprise a clamping element which is deformable radially to the outside relative to a longitudinal axis of the control valve housing by a force acting in actuating direction, for example. The clamping element is preferably disposed, in actuating direction, downstream of the actuating piston and connected workingly to the latter. At least some of the force needed to deform the clamping element may therefore be summoned up by the actuating piston.
According to a preferred embodiment, the deformable regions of the clamping element take the form of fingers, which extend parallel to the longitudinal axis of the control valve housing and all of which concentrically surround said longitudinal axis. The fingers in turn may be connected at their ends facing the vacuum chamber to a common carrier part, which is disposed downstream of the actuating piston.
A component disposed e.g. between the actuating piston and the clamping element and provided with sloping surfaces may be used to deform the clamping element. Instead of providing a separate component having sloping surfaces, the sloping surfaces may also be disposed on the actuating piston radially at the outside.
The coupling device is actuated through cooperation of the sloping surfaces (e.g. conical surfaces) with the deformable regions of the clamping element e.g. in the form of fingers. For said purpose, a reduction of the distance between the component provided with sloping surfaces and the clamping element is normally required. The sloping surfaces are preferably inclined relative to a longitudinal axis of the control valve housing.
A resilient element is preferably disposed between the clamping element and the component provided with sloping surfaces in such a way as to counteract a convergence of both components. The control valve housing may have a step acting counter to the actuating direction as a stop for the component provided with sloping surfaces. A sensing disk, which cooperates with a reaction disk disposed between a reaction piston of the vacuum brake booster and the control valve housing, is preferably disposed in actuating direction downstream of the clamping element.
The clamping means may further comprise an extension, which is coupled in actuating direction to the as yet uncoupled component of the magnetic module, cooperates with the clamping element and radially concentrically surrounds the outside of the clamping element at least in sections. In the case of radially outward deformation of the clamping element, the deforming regions of the clamping element come into abutment with the portion of the extension radially surrounding the outside of the clamping element. The coupling of said components to the actuating piston is effected as a result of the clamping element coming into abutment with the extension of the as yet uncoupled component of the magnetic module. The extension is preferably preloaded in actuating direction relative to the component of the magnetic module to be coupled, in order to allow a certain relative motion between said component and the extension.
The as yet uncoupled component of the magnetic module is preferably connected by a screw connection in an axially adjustable manner to a holding device. The holding device in turn may be preloaded by means of a resilient element counter to the actuating direction towards a step of the control valve housing. The step of the control valve housing prevents a movement of the component to be coupled of the magnetic module counter to the actuating direction. The resilient element, on the other hand, allows a certain displacement of the component to be coupled of the magnetic module in actuating direction.
The holding device for the uncoupled component of the magnetic module is preferably connected rigidly counter to the actuating direction and resiliently in actuating direction to the extension cooperating with the clamping element. To said end, the holding device may, for example, be provided radially at the inside with a groove. The extension may have a collar projecting into the groove; a resilient element disposed between a side surface of the groove facing the vacuum chamber and an opposing face end of the collar advantageously preloads the holding device and the extension towards one another.